“The Unknown Girl,” the latest film by the Belgian writer-directors Jean-Pierre and Luc Dardenne, is about the consequences of trying to do the right thing. By a logic at once elusive and meticulous, it is also, therefore, about the consequences of inaction. Moral indifference is an easy path, and it leads to other, graver sins, including cruelty and dishonesty. But the harder road of decency has its own traps and byways, as a young doctor named Jenny Davin discovers.

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A preview of the film.Published OnSept. 6, 2017CreditImage by Internet Video Archive

Jenny, played with somber concentration by Adèle Haenel, is devoted to her profession and her patients. She is thorough and compassionate, scolding her intern (Olivier Bonnaud) for failing to keep his emotions in check and communicating clearly and efficiently with the mostly poor people who come to her office. On the verge of moving into a more prestigious position, she decides, almost impulsively, to stay where she is, and take over her retired mentor’s small general practice. It’s what she loves, and she is loved in return.

But in the Dardennes’ moral universe the ways of love are never simple. Having established that Jenny is a good person, the film allows her what at first seems to be a trivial lapse. Late one evening, long after office hours have finished, the front buzzer sounds and Jenny decides to ignore it. (That buzzer and the functional ringtone on Jenny’s cellphone are the film’s principal sound effects and the closest thing it has to a musical score.) The next morning, a woman is found dead nearby, and security video identifies her as the person who had been at Jenny’s door.

Not much else is known about her, and Jenny, in a fit of self-reproach, tries to compensate for her neglect by finding out what she can. As they often have before — in “The Son,” “The Child” and “Lorna’s Silence,” most notably — the Dardennes use techniques of suspense to pursue an ethical problem. And vice versa. Jenny, stubborn in the face of her own fear and the threats and evasions of the people she encounters, is equal parts detective and priest. She uses the promise of doctor-patient confidentiality, a secular version of the seal of confession, to coax out shameful secrets. She also burrows into a demimonde of poverty, desperation and crime that exists just beyond her waiting room and sometimes spills into it.

“The Unknown Girl” is as tense as a police procedural, and as mysterious as a religious parable. It’s a Dardenne brothers film, in other words, shot with unassuming lyricism and an eye for everyday detail and full of modest, eloquent performances (including from Jérémie Renier, the star of “The Child.”) The brothers’ methods are austere, but not harsh. They are students of human cruelty and champions of the resistance to it. In their world — in our world, as their unforgettable films reveal it to us — there is no end of guilt, and just enough grace.